Mistakes in the Drake Equation

byPaul GilsteronFebruary 16, 2018

Juggling all the factors impacting the emergence of extraterrestrial civilizations is no easy task, which is why the Drake equation has become such a handy tool. But are there assumptions locked inside it that need examination? Robert Zubrin thinks so, and in the essay that follows, he explains why, with a particular nod to the possibility that life can move among the stars. Although he is well known for his work at The Mars Society and authorship of The Case for Mars, Zubrin became a factor in my work when I discovered his book Entering Space: Creating a Spacefaring Civilization back in 2000, which led me to his scientific papers, including key work on the Bussard ramjet concept and magsail braking. Today’s look at Frank Drake’s equation reaches wide-ranging conclusions, particularly when we begin to tweak the parameters affecting both the lifetime of civilizations and the length of time it takes them to emerge and spread into the cosmos.

by Robert Zubrin

There are 400 billion other solar systems in our galaxy, and it’s been around for 10 billion years. Clearly it stands to reason that there must be extraterrestrial civilizations. We know this, because the laws of nature that led to the development of life and intelligence on Earth must be the same as those prevailing elsewhere in the universe.

Hence, they are out there. The question is: how many?

In 1961, radio astronomer Frank Drake developed a pedagogy for analyzing the question of the frequency of extraterrestrial civilizations. According to Drake, in steady state the rate at which new civilizations form should equal the rate at which they pass away, and therefore we can write:

Equation (1) is therefore known as the “Drake Equation.” Herein, N is the number of technological civilizations is our galaxy, and L is the average lifetime of a technological civilization. The left-hand side term, N/L, is the rate at which such civilizations are disappearing from the galaxy. On the right-hand side, we have R∗, the rate of star formation in our galaxy; fp, the fraction of these stars that have planetary systems; ne, is the mean number of planets in each system that have environments favorable to life; fl the fraction of these that actually developed life; fi the fraction of these that evolved intelligent species; and fc the fraction of intelligent species that developed sufficient technology for interstellar communication. (In other words, the Drake equation defines a “civilization” as a species possessing radiotelescopes. By this definition, civilization did not appear on Earth until the 1930s.)

By plugging in numbers, we can use the Drake equation to compute N. For example, if we estimate L=50,000 years (ten times recorded history), R∗ = 10 stars per year, fp = 0.5, and each of the other four factors ne, fl, fi, and fc equal to 0.2, we calculate the total number of technological civilizations in our galaxy, N, equals 400.

Four-hundred civilizations in our galaxy may seem like a lot, but scattered among the Milky Way’s 400 billion stars, they would represent a very tiny fraction: just one in a billion to be precise. In our own region of the galaxy, (known) stars occur with a density of about one in every 320 cubic light years. If the calculation in the previous paragraph were correct, it would therefore indicate that the nearest extraterrestrial civilization is likely to be about 4,300 light years away.

But, classic as it may be, the Drake equation is patently incorrect. For example, the equation assumes that life, intelligence, and civilization can only evolve in a given solar system once. This is manifestly untrue. Stars evolve on time scales of billions of years, species over millions of years, and civilizations take mere thousands of years.

Current human civilization could knock itself out with a thermonuclear war, but unless humanity drove itself into complete extinction, there is little doubt that 1,000 years later global civilization would be fully reestablished. An asteroidal impact on the scale of the K-T event that eliminated the dinosaurs might well wipe out humanity completely. But 5 million years after the K-T impact the biosphere had fully recovered and was sporting the early Cenozoic’s promising array of novel mammals, birds, and reptiles. Similarly, 5 million years after a K-T class event drove humanity and most of the other land species to extinction, the world would be repopulated with new species, including probably many types of advanced mammals descended from current nocturnal or aquatic varieties.

Human ancestors 30 million years ago were no more intelligent than otters. It is unlikely that the biosphere would require significantly longer than that to recreate our capabilities in a new species. This is much faster than the 4 billion years required by nature to produce a brand-new biosphere in a new solar system. Furthermore, the Drake equation also ignores the possibility that both life and civilization can propagate across interstellar space.

So, let’s reconsider the question.

Estimating the Galactic Population

There are 400 billion stars in our galaxy, and about 10 percent of them are good G and K type stars which are not part of multiple stellar systems. Almost all of these probably have planets, and it’s a fair guess that 10 percent of these planetary systems feature a world with an active biosphere, probably half of which have been living and evolving for as long as the Earth. That leaves us with two billion active, well-developed biospheres filled with complex plants and animals, capable of generating technological species on time scales of somewhere between 10 and 40 million years. As a middle value, let’s choose 20 million years as the “regeneration time” tr. Then we have:

where N and L are defined as in the Drake equation, and ns is the number of stars in the galaxy (400 billion), fg is the fraction of them that are “good” (single G and K) type stars (about 0.1), fb is the fraction of those with planets with active biospheres (we estimate 0.1), fm is the fraction of those biospheres that are “mature” (estimate 0.5), and nb, the product of these last four factors, is the number of active mature biospheres in the galaxy.

If we stick with our previous estimate that the lifetime, L, of an average technological civilization is 50,000 years, and plug in the rest of the above numbers, equation (2) says that there are probably 5 million technological civilizations active in the galaxy right now. That’s a lot more than suggested by the Drake equation. Indeed, it indicates that one out of every 80,000 stars warms the home world of a technological society. Given the local density of stars in our own region of the galaxy, this implies that the nearest center of extraterrestrial civilization could be expected at a distance of about 185 light years.

Technological civilizations, if they last any time at all, will become starfaring. In our own case (and our own case is the only basis we have for most of these estimations), the gap between development of radiotelescopes and the achievement of interstellar flight is unlikely to span more than a couple of centuries, which is insignificant when measured against L=50,000 years. This suggests that once a civilization gets started, it’s likely to spread. Propulsion systems capable of generating spacecraft velocities on the order of 5 percent the speed of light appear possible. However, interstellar colonists will probably target nearby stars, with further colonization efforts originating in the frontier stellar systems once civilization becomes sufficiently well-established there to launch such expeditions.

In our own region of the galaxy, the typical distance between stars is five or six light years. So, if we guess that it might take 1,000 years to consolidate and develop a new solar system to the point where it is ready to launch missions of its own, this would suggest the speed at which a settlement wave spreads through the galaxy might be on the order of 0.5 percent the speed of light. However, the period of expansion of a civilization is not necessarily the same as the lifetime of the civilization; it can’t be more, and it could be considerably less. If we assume that the expansion period might be half the lifetime, then the average rate of expansion, V, would be half the speed of the settlement wave, or 0.25 percent the speed of light.

As a civilization expands, its zone of settlement encompasses more and more stars. The density, d, of stars in our region of the galaxy is about 0.003 stars per cubic light year, of which a fraction, fg, of about 10 percent are likely to be viable potential homes for life and technological civilizations. Combining these considerations with equation 2, we can create a new equation to estimate C, the number of civilized solar systems in our galaxy, by multiplying the number of civilizations N, by, nu, the average number of useful stars available to each.

For example, we have assumed that the average lifespan, L, of a technological species is 50,000 years, and if that is true, then the average age of one is half of this, or 25,000 years. If a typical civilization has been spreading out at the above estimated rate for this amount of time, the radius, R, of its settlement zone would be 62.5 light years (R = VL/2 = 62.5 ly), and its domain would include about 3,000 stars. If we multiply this domain size by the number of expected civilizations calculated above, we find that about 15 billion stars, or 3.75 percent of the galactic population, would be expected to lie within somebody’s sphere of influence. If 10 percent of these stars are actually settled, this implies there are about 1.5 billion civilized stellar systems within our galaxy. Furthermore, we find that the nearest outpost of extraterrestrial civilization could be expected to be found at a distance of 185-62.5 = 122.5 light years.

The above calculation represents my best guess as to the shape of things, but there’s obviously a lot of uncertainty in the calculation. The biggest uncertainty revolves around the value of L; we have very little data to estimate this number and the value we pick for it strongly influences the results of the calculation. The value of V is also rather uncertain, although less so than L, as engineering knowledge can provide some guide. In Table 1 we show how the answers might change if we take alternative values for L and V, while keeping the other assumptions we have adopted constant.

Table 1 The Number and Distribution of Galactic Civilizations

V=0.005 c

V=0.0025 c

V=0.001 c

L=10,000 years

N (# civilizations)

1 million

1 million

1 million

C (# civilized stars)

19.5 million

2.4 million

1 million

R (radius of domain)

25 ly

12.5 ly

5 ly

S (Separation between civilizations)

316 ly

316 ly

316 ly

D (distance to nearest outpost)

291 ly

304 ly

311 ly

F (fraction of stars within domains)

0.048%

0.006%

0.0025%

L=50,000 years

N (# civilizations)

5 million

5 million

5 million

C (# civilized stars)

12 billion

1.5 billion

98 million

R (radius of domain)

125 ly

62.5 ly

25 ly

S (Separation between civilizations)

185 ly

185 ly

185 ly

D (distance to nearest outpost)

60 ly

122.5 ly

160 ly

F (fraction of stars within domains)

30%

3.75%

0.245%

L=200,000 years

N (# civilizations)

20 million

20 million

20 million

C (# civilized stars)

40 billion

40 billion

18 billion

R (radius of domain)

500 ly

250 ly

100 ly

S (Separation between civilizations)

131 ly

131 ly

131 ly

D (distance to nearest outpost)

0 ly

0 ly

31 ly

F (fraction of stars within domains)

100%

100%

44%

In Table 1, N is the number of technological civilizations in the galaxy (5 million in the previous calculation) , C is the number of stellar systems that some civilization has settled (1.5 billion, above), R is the radius of a typical domain (62.5 ly above), S is the separation distance between the centers of civilization (185 ly above), D is the probable distance to the nearest extraterrestrial outpost (122.5 ly, above), and F is the fraction of the stars in the galaxy that are within someone’s sphere of influence (3.75% above).

Examining the numbers in Table 1, we can see how the value of L completely dominates our picture of the galaxy. If L is “short” (10,000 years or less), then interstellar civilizations are few and far between, and direct contact would almost never occur. If L is “medium” (~50,000 years), then the radius of domains is likely to be smaller than the distance between civilizations, but not much smaller, and so contact could be expected to happen occasionally (remember, L, V, and S are averages; particular civilizations in various localities could vary in their values for these quantities). If L is a long time (> 200,000 years), then civilizations are closely packed, and contact should occur frequently. (These relationships between L and the density of civilizations apply in our region of the galaxy. In the core, stars are packed tighter, so smaller values of L are needed to produce the same “packing fraction,” but the same general trends apply.)

Any way you slice it, one thing seems rather certain: There’s plenty of them out there.

As I’ve said on other posts, I believe that commonality and complexity of life are inversely proportional. In other words, civilized sentients should be rare, microbes should be common, with complex plants/animals somewhere in between. This is the pattern seen on earth, and I think it makes sense to extrapolate this to the wider universe.

As for the Fermi Paradox, I think it’s important to note that we have not taken an exhaustive survey of the Milky Way Galaxy, let alone the universe. In fact, our search for ETI has been very short and incomplete; they may be out there but undetected. We can safely rule out the idea that our galaxy is teeming with advanced interstellar civilizations e.g. Star Trek, but almost anything less than this extreme is possible. There could be an alien civilization on the other side of the galactic core where it cannot be seen. Maybe there are alien civilizations who have turned inward into a virtual reality like the Matrix, rather than outward towards the physical universe; this could leave them confined to a single planet and difficult to detect. An even more plausible scenario is that the nearest alien civilization will not be our contemporary — our first contact may be with ruins and artifacts built eons ago.

Given these possibilities, there is still plenty of potential for ETI. The fact that we can rule out the most extreme iterations does not negate this in any way, especially given my expectation that sentience and civilization should be relatively uncommon, seeing as they arose only once on earth (as far as we know).

Overall, there is no reason to be pessimistic towards the potential for ETI. I think this article does a good job of improving the Drake Equation; hopefully we will be able to fine-tune it further.

I am glad you brought up the fact that humanity has done relatively little in the search for alien life, intelligent and otherwise. It just seems like we’ve done a lot to find our cosmic neighbors due to the strong public clamor for it and all those science fiction versions of our thoughts on aliens.

Yes, “modern” SETI has been happening since 1960 when Frank Drake ramped up Project Ozma, which looked at two nearby Sol-type stars for all of two months via radio telescope, only to discover a terrestrial military frequency.

However, SETI has been sporadic for most of its existence, even with the big deal made over Breakthrough Listen and the initial infusion of cash (has it been replenished yet?). Even the few SETI programs that were kept running for years, such as the one using the Big Ear radio telescope at Ohio State University (OSU) which made the famous Wow! signal discovery in 1977, were run by volunteers who had to split their time between SETI and their paying day jobs.

The Wow! signal wasn’t even noticed until hours after it had been detected when a member looked at the computer printout, far too late to follow up on the signal and one that with today’s SETI techniques would probably have been dismissed as a false signal. However, since that did not happen on that day in August over forty years ago, it has been left as a mystery ever since, one that the media always turns to as a prime example of a possible alien signal.

The other issue with SETI is that most of its history has been dominated by scientists who focused on radio frequencies coming from Earthlike planets circling Sol-type stars – in other words, they looked for beings who would behave a lot like us. Yes, SETI has branched out in the last twenty years, first with Optical SETI and then some still-token efforts for advanced technological activities such as Dyson Shells, but again, these are still the exception and not the rule – and if you think some scientists are sitting at their consoles watching the skies 24/7 for ETI, think again.

See here for the real history of our SETI efforts, the one that doesn’t get delved into enough:

As for searching for less advanced biological life in our own Sol system, only the Viking missions to Mars in 1976 were truly dedicated to such an effort. When their results were declared ambiguous due to a curious lack of first studying the material properties of the Martian surface before digging for alien microbes, NASA shied away from further deliberate exobiological expeditions for decades. Even the rovers sent to the Red Planet much later on were explicitly designated only for finding ancient signs of water and organic chemistry. Formations that looked suspiciously like fossils were shown but conspicuously not expanded upon or given further pursuit.

And yes, there are Europa and Enceladus with their amazing bounties of subsurface water along with Titan and its exotic and prebiotic environment, but any serious exploration expeditions to find native life there are decades away.

So, yeah, we really have done far more talk, speculation, and “play acting” about alien life compared to actively searching for it. What we have done so far is better than nothing, but we could and should be doing a whole lot more. This is why those who say that because we haven’t had ETI knocking on our door they therefore do not exist are being incredibly premature.

The Wow! Signal is intriguing, but it also serves as a good example of SETI’s limitations. The Big Ear telescope could only be adjusted for declination, and used the Earth’s rotation to scan the sky. The Wow! signal is based on the time-window during which the telescope happened to scan across the relevant patch of sky.

While this is an anecdote, I think it demonstrates how limited our SETI efforts have been. There’s also the issue of other frequencies, the relatively short length of time we’ve searched, and the fact that we haven’t proven or disproven the presence of alien life in our solar system.

So far, all we’ve done is briefly shine small beams of light around a cavernous dark room, and then conclude that there’s nothing of interest within.

“I believe that commonality and complexity of life are inversely proportional. In other words, civilized sentients should be rare, microbes should be common.”

Yes that is exactly my opinion also.
I believe that life get started quite often when conditions permit.
The first bloc if that is the word, is if that life is able to become complex such as having efficient metabolism with enzymes or if they get mitochondria or any equivalent.
The next point where things could stop is if this life go trough a cambrian revolution or not. On a world pelted by comets or rocks this life might never have a chance to get further, same problem on a world where the star is more irregular than our Sun – which in one unusually benign star.

When there’s complex life, intelligence seem to be common.
Parrots and many other birds have the intelligence of a 4-year human, or better. My bet for birds is Siberian jay, it is not an imitating bird who has a very complex ‘language’ it use when it approach a human looking for a snack. Mammals even more, but research is lacking for many, I and some others consider the common Brown bear to be extraordinary intelligent. They understand a doorhandle use it to walk into peoples houses to look for food, and also understand the need for refrigeration – placing surplus food in cold springs, and in snow to preserve it.

So intelligence happen many times in complex life, but they had many chances to become a civilisation before humans did. So I consider such unusual.

With the distance and time involved I think we have a better chance of finding ruins of a civilisation than to shake hands, like in the film Valentian! :)
So when KIC 8462852 come under discussion, I rather though for myself it might be for a dead race and their big structure falling apart, dust and collisions creating fragment that caused the change in brightness.
But I do not think I mentioned it, talk about other civilisations and the possibility if they exist now always lead to UFO believer posts. So I stop now.

There are a number of ‘filters’ for life and civilization, including: habitable planet, stable biosphere, emergence and survival of intelligence, emergence and survival of civilization, etc.

For the majority of earth’s history, life consisted of microbes and bacterial mats. Gradually, multicellular life evolved, diversifying greatly during the Cambrian Explosion (and the mysterious Ediacaran period shortly before). Even then, it took many more millions of years for plants and animals to evolve and establish complex ecosystems. These were punctuated by extinction events, forcing life to rebuild each time.

As it happened, sentient intelligence and civilization were finally achieved by mammals, although perhaps it could’ve happened another way. As you point out, other animals have intelligence too (EQ or encephalization quotient is the best measure we have thus far) but none made the breakthrough to civilization. One could argue that the social insects (ants, bees, termites, etc) are a kind of civilization, or maybe a sentient intelligence (if you consider a colony to be an emergent individual). However, their technology is very rudimentary, and they have not achieved mastery of their environment as humans have. It is interesting to note that eusociality is a rare phenomenon — among the countless species of insects, only a few have made that breakthrough. This is consistent with my hypothesis that intelligence/civilization is a rare phenomenon. There are also intervening factors between intelligence and civilization, e.g. being unable to control fire or other chemical reactions — this is true of both social insects, and intelligent ocean life such as dolphins. Overall, I expect the many filters of evolution to maintain my model of common simplicity and rare complexity.

I think it’s possible to discuss exotic ideas like ETI and alien civilizations in a sober, rational manner, while not including UFOs and other paranormal ideas. It’s a thin line at times, but this blog successfully maintains it.

And thank you for your very interesting reply. Eusociality is indeed very rare, the only example that popped into my mind when you mentioned it were one case of fish who do it – though only during the mating period.
But it could indeed be a first step toward a civilisation.
Now as for social insects and with potential intelligence, there actually were a professor who did mention ants at a meeting we held last year.
I looked at him closely, expecting him to be joking.
He were not.
But that idea about ants or bee’s having a sort of collective intelligence is long disproved.
And I pointed out to him that if there actually were anything such, their use of pheromones would make the information pace so slow that it certainly never would qualify as ‘intelligence’ in the way we define it.
But there’s no rudimentary technology here, we only see examples of early stone age tech among a few birds and simians (use of rocks and sticks) and for mammals among otters, beavers and bears.

And yes there’s many filters, I didn’t even mention that humans have created so many civilisations without going high tech in history. From well known examples of Babylon and Egypt to less known ones like the ‘Indians’ in north America who once had a sprawling city of a million inhabitants who were so completely eradicated by biological warfare, genocide and then followed by history falsification to such a degree that it blew my mind when some archeologists actually were allowed to publish the results from their dig a few years ago.

And there we have another filter. When politics and economical interests (also religion or superstition) is allowed to meddle with science, things could go very bad indeed. That is a filter that can have dire consequences and made the smartest species fail even when they are so close to wonderful achievements.
So that’s a filter some have right in front of their eyes, without any reaction – a bad sign indeed.

This is also the reason I react strongly to suggestions that Oumuamua would be some kind of alien spacecraft.
If we are to at least upheld the idea of talking about things that are of the real world here we need to follow the simple rule of thumb that: ‘Extraordinary claims need extraordinary evidence.”
So that’s why I react strongly to such UFO talk as seen in the comments about Oumuamua, and hinted this forum need stronger moderation.

I’ll admit, collective colony intelligence is a stretch, but it is a possible interpretation. Of course, their use of pheromones is far less efficient than synapses, just as a mechanical ‘difference engine’ is far less efficient than a computer. But given how effectively a colony operates as a cohesive unit, I don’t think the idea should be dismissed. And in fact they do have rudimentary technology, e.g. fungus farms in leaf-cutter ant colonies.

Yes, Cahokia was a significant culture with complex organization, trade routes, and urban centers. Due to the impact of disease (first and foremost) and then the violence of colonization, the Native American societies collapsed and were extirpated; so much so that many people have never heard of Cahokia. This touches on a larger point — just as life has had to recover from extinction events, civilization has gone through upheavals and collapses. One could draw an analogy between the fall of Rome and the extinction of the dinosaurs, as well as between the Late Bronze Age collapse and the Permian-Triassic extinction event.

Science is a major component of modern civilization, alongside industrialization and the Enlightenment. I believe the greatest threat to both science and enlightenment ideals is the rise of fanaticism and fundamentalist ideology. Fundamentalism has gained power and influence lately, not only in the Middle East, but within the West as well. It is a cause for concern.

I agree, extraordinary claims require extraordinary evidence, this is an excellent principle. I think the key is to start from a scientific foundation and apply rational thought — by sticking to that methodology, one can go on flights of speculation while still avoiding pseudoscience and the paranormal.

“At Seti we want to look at as many frequency channels as we possibly can because we don’t know what frequency ET will be broadcasting on and we want to look for lots of different signal types – is it AM or FM, what communication are they using?” said Dr. Dan Werthimer, chief scientist at SETI.

The recent price rise of digital currencies, though, brought with it a new wave of people who are interested in mining. As a result, the demand for GPUs has skyrocketed in recent months and created a scarcity of GPUs in the marketplace.

“We’d like to use the latest GPUs and we can’t get [them],” said Dr. Werthimer, “That’s limiting our search for extraterrestrials, to try to answer the question, ‘Are we alone? Is there anybody out there?’. This is a new problem, it’s only happened on orders we’ve been trying to make in the last couple of months.”

“We’ve got the money, we’ve contacted the vendors, and they say, ‘We just don’t have them’,” he added.

We can play with the Drake Equation all we want, but if people are not going to make the effort to search, or hamper those who are trying to find aliens, then it will remain an intellectual exercise at best.

Hopefully the SETI folks quickly bought NVIDIA stock at a good time to increase the value of their funding. ;)

But seriously, the shortage of GPU boards because of cryptocurrency mining is going to be seen as a folly in the near future. Did you see that Russian scientists were using a supercomputer for nuclear research to do BitCoin mining? If you see your computer slowing down, that might be because coin mining software has hijacked the browser to earn publishers income instead of pushing ads. I hope that anti-virus software stops that dead soon.

…And human beings look down upon crows for coveting shiny things, when they have the same “affliction” (gold, silver, diamonds, and now shining pixels on computer screens)–at least the Dutch tulips were pretty, and relaxing to smell and look at… Jonathan Swift’s Houyhnhnms would never have built radio telescopes or lasers (nor would even have wanted to), but would have been more cultured and sensible beings.

Crows at least have the decency not to pretend to be more than they are.

However, so far as I know, they have not bothered to search for alien life, either. So maybe a bit of overreaching by a species is in order to be more than what nature programmed us with. Otherwise, why bother having a technological civilization at all?

We’ll have something to say when we have real data. Number of star systems and planets surveyed, number of planets harboring any form of life, number of planets with something we recognize as sentient, and so on. Then we can get some kind of realistic number from the Drake equation and not before.

The project is the brainchild of Danny Hillis, an engineer-inventor who started up the Long Now Foundation in the year 01996 with the vision of fostering long-term thinking — and creating a timepiece that gives humanity a cosmic sense of scale for generations to come.

“I want to build a clock that ticks once a year,” Hillis wrote. “The century had advances once every 100 years, and the cuckoo comes out on the millennium. I want the cuckoo to come out every millennium for the next 10,000 years.”

Bezos was taken with the idea, and agreed to fund the project and provide the land.

“Over the lifetime of this clock, the United States won’t exist,” Bezos told Wired back in 2011. “Whole civilizations will rise and fall. New systems of government will be invented. You can’t imagine the world — no one can — that we’re trying to get this clock to pass through.”

Ljk you have brought up a very good point and there is a man-made clock already over 10’000 years old – Gobekli Tepe. This may represent how we would discover an alien race, these temples where covered by later generations garbage, and in doing so left them in an amazingly good condition. The pyramids would eventually weather away but buried structures could last for billions of years. (Aka 2001)

Most of our urban infrastructure would collapse and either be overgrown or buried within a matter of centuries. The Great Pyramids of Egypt will completely weather away in about 125,000 years, if I recall from another source correctly.

Space offers our best hope for preserving our artifacts. It has been estimated that the side of the Voyager Interstellar Records facing outwards from the space probes will last about one billion years if nothing major happens, while the record side facing inwards will last much longer. Satellites circling Sol or in Earth’s geosynchronous orbit should last for millions of years. See here:

I found those initiatives so pessimistic and “unfuturistic” or conservatist… Instead of dedicating efforts to spread human civilization outside Earth, they strive to preserve current culture as a relic, as if we would never reach space and perish down here sooner than later. As if the status quo is not changing and will never change. An unrealistic and depressing view of things.

The absence of evidence is not the evidence of absence so we can’t really rule out anything especially ETs with relativistic and FTL travel. Just because they don’t announce there presence does not mean they don’t know about us.

Also the extinction of human life through nuclear war is a possibility. If we wanted to destroy ourselves we in potential could considering all of the nuclear weapons in the world.
Even a full nuclear exchange in only the northern hemisphere would be disasterous. The ozone layer would be reduced by 30 to 70 percent causing crops to fail because the heat of expanding plasma sphere created by x rays from a H bomb explosion has a temperature of 55 to 10 million degrees which burns the nitrogen in our atmosphere and will oxidize it to form nitric oxide. One NO molecule can destroy ten to twelfth or ten to the thirteen power of ozone molecules.

The science which supports nuclear winter is sound. The mount Tambora volcano eruption of 1815 put one million pounds of dust into the atmosphere and there was no summer that year due and the crops failed due to the reduced sunlight and temperature. A nuclear war of one hundred Hiroshima yield bombs would put five million pounds of dust into the upper atmosphere and reduce sunlight for ten years. Scientific American article: “The Nuclear Doomsday Lock Still Clicks.” physicist Laurence Krauss, Scientific American, January 2010 The dust from this would be less than a full nuclear northern exchange since the weapons would be higher yield H bombs.
Then of course there is the radiation, radioactive fallout, the huge fire storms resulting from heat of so many H bomb blasts and radiation burns from the x ray and gamma rays from the flash of the explosions. There would be no food in the northern hemisphere. Nothing would grow and what did grow would be contaminated with radioactivity. One would have to grow crops underground. Cesium 137 and other radioactive fallout does cause cancer.

Let’s hope that Kim Jong Un can’t “weaponize” that NO molecule ozone molecules destruction statistic. NO not only destroys ozone molecules, but it may also do so *without* being “consumed” itself (as a catalyst, in other words–satelloids moving at the edge of the atmosphere might utilize this property of NO with the monatomic oxygen up there, for propulsion), and:

NO acts in this way when it causes monatomic oxygen (O) to convert back into normal diatomic oxygen (O2), with a great release of energy, as a highly-luminous March 1956 Aerobee night firing showed. The sounding rocket released 18 pounds of NO 66 miles up, which produced a bright “star” that spread until it was three miles across. NO released from ICBMs or satellites (or perhaps produced by suitably “salted” high-altitude nuclear detonations) during the day could let dangerous amounts of solar UV radiation reach the ground, harming people, animals, and plants (including crops).

The paper that he links doesn’t prove what he says. On the contrary, as I said before, it proves that the effect is mostly restricted to northern hemisphere, and have even less effect on ocean’s climate.

I agree on the point that life potentially can get started more than once in a stellar system. I am optimistic on this already in our solar system, with 2 potential worlds that might harbour life, and at least one that might be prebiotic now but where life might get started, ie: Titan.

When the planets in the Trappist1 system were found we even got an example where 3 planets at once were potentially habitable.
Having the smaller planet d. on the inner side of the habitable zone is actually one advantage since it would loose more volatiles, while the larger planet d. might have a thick atmosphere providing a greenhouse effect.
They probably have bound orbits showing one side to the star, we do not know if any of these planets have a large moon though. It would not be seen in current data, but if there is one it would need to have a very tight orbit not to be perturbed by the close proximity of the other planets.

Anyway I have recently recalculated my own estimate in the Drake Equation.
My result seem to show I am more optimistic on the presence of life, but less so on how often that rise to complex life.
One other difference is that I am a bit less optimistic than most on how often one intelligent species create a civilisation and go technological. (Would dolphins, Elephants or the best example: Parrots or Ravens have done so in case humans had not beat them to it? I am not certain they would, intelligence need not go high tech on automatic.)

Anyway, I end up with a value for N of 40 civilisations present in the galaxy right now.
And I find that result to be so close to say that we’re in agreement. :)

As a sidenote: I have still not made up my mind if your proposed and continuously exploding Uranium Tetrabromide salt water engine should be labelled the best idea ever (12,9 MN / 425 GW thrust), or it should be labeled Mad Science. :)

Now of course this will depend on in which manner we find aliens, what they are like, and how they may respond to us, but as for the IDEA of encountering extraterrestrials, the concept has been with us for so long now culturally that the very thought itself may not send humanity into a massive panic as once thought.

Caleb Scharf is the director of astrobiology at Columbia University in New York City. Since receiving his PhD in astronomy from the University of Cambridge in 1994, he has become an internationally-known scientist, lecturer, and writer—the author of more than 100 scientific papers and the winner of the American Astronomical Society’s 2011 Chambliss Award for excellence in writing. His research includes the study of exoplanets and the search for life on other worlds; his latest book is “The Zoomable Universe.”

I spoke to Dr. Scharf about the ongoing pursuit of Earth-like planets in the galaxy, and the possibility of life elsewhere, particularly in the solar system. Here are excerpts from our conversation.

CS: Honestly, it depends on the day of the week. I am constantly assessing and reassessing my ideas on these questions. Today I think there is a fair chance there’s lots and lots of life, both simple and intelligent—that is, intelligent with some technological characteristics.

But we have underestimated the difficulty in spotting life. After all, we still don’t know the full contents of our own solar system, and we’re only just entering the era of really big data for astronomy. Having said that, I’m almost certain humans are effectively unique—our precise history and evolutionary pathway has probably not happened anywhere else.

It’s very good to see the regeneration argument finally gaining attention. It’s just the high-tech state that is fragile, humans as a species are extremely hardy. Surely, a supervolcano eruption can reduce us to the stone age, but I doubt that even a K-T-scale impact will wipe all stone-age-level populations completely, and regenerative development, with many artifacts still there, will take much less time than the Dawn Age.

What holds my mind is some kind of anti-copernician principle as an argument. Our place in the universe may be quite usual, and we think that interstellar colonization as a very hard challenge, but still a manageable one. But if the civilization formation rate is high, than there are extreme cases too – more lucky ones, or more motivated ones, or both. Some develop in systems with two or more readily habitable planets, and do not encounter a disappointment stage between space race and space industry – imagine what would we have achieved by now if Venus or Mars was truly habitable! Some others develop in wide binary stellar systems, where there is another star a thousand AU away, with it’s own habitable system. Some live on super-earths or(and) in red dwarf systems, with higher escape and transfer velocities – the guys that cannot reach orbit without some kind of nuclear or advanced beam-powered propulsion, and yet won’t give up because a second habitable planet shines at them at nights. They won’t stuck at chemical propulsion stage. And definitely there were civilizations through the ten-billion-year-long history of Local Cluster with a real survival motivation too. Some beings who had to move to the stars because of the coming disaster which would render the whole system inhabitable, like a passage of a brown dwarf through the system, with kicked their gas giant into a homeworld-crossing orbit and gave them several hundred years of relatively stable conditions, but afterwards imminent chaos and destruction. And some freak cases which we can not imagine, when it was like “go to the nearest stars with habitable worlds in the next two hundred years, without cyborgization and other near-singularity things, or die” – if even one of such civilizations managed to actually get to the next habitable world and start an iterative colonization, they will be everywhere.

This leads to the classic formulation, but reinforced with the line like “within the reach of colonization waves powered by thermonuclear-powered flight, there likely were civilizations whose motivation for interstellar travel was many orders of magnitude stronger than ours”.

I guess, the likely explanation is the “they turn to the deep”, formulated like “all intelligent life necessarily leaves the exponentially extensive path, in terms of used matter and harnessed energy, before reaching massive astroengineering stage”…

New research in to how Earth’s atmosphere evolved over time could hold the key to detecting life on exoplanets, according to scientists from the University of St Andrews and Cornell University.

The new study, published in The Astrophysical Journal, details how Earth’s atmosphere evolved over time and how this corresponds to the appearance of different forms of life.

The team, led by Dr. Sarah Rugheimer, astronomer and astrobiologist from the School of Earth and Environmental Sciences at the University, studied different geological epochs from Earth’s history, modelling the atmospheres around different stars, bigger and smaller than our Sun. The researchers found that a planet’s star type is an important factor in how an exoplanet’s atmosphere develops and in how detectable signs of life, aka biosignatures, will be.

Zubrin’s approach, while quite interesting–I enjoyed putting together a spreadsheet using his modifications–is too optimistic in some areas and perhaps too pessimistic in others.

The leveraging power of fossil fuels should not be underestimated in terms of jump-starting a modern, high-tech civilization. In this short essay, high tech = widespread use of electricity to power electrical devices. Every nuclear power plant, electric wind turbine and solar panel on Earth has been built using the fossil fuel crutch. Without access to a large quantity of high-quality and easy-to-reach fossil fuels, the development of a high-tech civilization almost certainly would be much more difficult to achieve. Indeed, it may be next to impossible. For example, it might require long-term stability that by and large has not occurred on the Earth, such as a less varied climate, so that agrarian societies have a longer mean lifespan, allowing a long period of relative prosperity that may be supportive of the gradual development of key electrical technologies.

In any event, once the vast solar battery that is fossil fuels is used up, it appears to take a very long time to rebuild the reserves. Given the length of time it has taken for the Earth’s supply to be created through various biological and geological processes, resupplying the battery is on the order of hundreds of millions of years. Thus, I believe that Zubrin’s 20 million years for a mean time between civilization restarts is much too short, by about an order-of-magnitude.

If you put a 200 ma separation between new civilizations on the same world, then 5,000,000 civilizations in the galaxy becomes 500,000. This results in a mean distance between each interstellar civilization’s center of around 400 light years and puts 0.4% of the galaxy’s stars within the sphere of influence of each civilization.

Furthermore, if a civilization uses up much of its supply of fossil fuels, followed by a destructive global thermonuclear war, then it is not likely that a modern high-tech civilization will be rebuilt any time soon afterward. It could happen, but this is not certain. So you still need that fraction of civilizations that self-destruct built into the equation. If a significant portion of civilizations self-destruct and are unable to rebuild until the fossil fuel battery is resupplied, then 50,000 years is a bit optimistic for a mean civilization lifetime. It might be more like 100-200 years. However, there may be the rare civilization that beats the odds and endures for a long time, say on the order of millions of years. If this happens say 1 in 10,000, and 100 years is assigned for the average age of the other 9,999, then one might assume a mean lifetime of 1,000 years.

Assigning 1,000 years to L, a figure that my in fact be optimistic, the number of extant civilizations drops to 10,000. This results in a mean distance of about 1,470 light years between centers of influence, and only a tiny fraction of stars within the sphere of influence of each.

Also, it is uncertain that every civilization would achieve, or even pursue, interstellar capability. Zubrin’s argument for the inevitability of such is weak. For a civilization faced with severe fossil fuel depletion, it may not be possible, or in the least desirable to pursue interstellar capability. There may be too many problems on the homeworld to support such long-term and far-thinking technologies. Fossil fuel depletion is known to trigger geopolitical positive feedback loops that have many negative consequences: wars between nations, civil unrest, revolutions, coups, each of which take a toll on the preexisting infrastructure, forcing out of necessity the diversion of available capital to things other than spaceflight.

On the flip side, Zubrin has chosen to ignore the “Red Empire.” This is the vast number of red dwarf stars in the galaxy. Current thinking with red dwarfs is that many could be supportive of active Earth-like biospheres. If you include a large fraction of red dwarfs as “good” stars, then fg might look more like 0.60, or even higher. Using 0.60 combined with my less optimistic numbers above, the equation spits out 60,000 civilizations, nearly all confined to one star with a mean separation of about 800 light years. Again, the issue with this, of course, is that it only takes one civilization to beat the odds and endure for a very long time, say millions of years. This combined with interstellar travel would mean colonization of a large region of the galaxy–if not the entire galaxy–in short-order, geologically speaking.

Well, people without fossil fuels created pyramids, aqueducts and cathedrals with no fossil fuels at all. I find it highly unlikely that people from the same species and with current knowledge could not recreate current technology in less than a century.

Wood burning, or horse drawn, high tech civilization? Historically, wood burning resulted in deforestation and the eventual switch to fossil fuels, primarily coal. That switch would be cut off, preventing the use of cheap, abundant energy to power a civilization. You could recreate the Roman Empire or dynastic China, but not the modern world.

People of the past, without fossil fuels, created very sofisticated technological civilizations, not as advanced as ours, but nevertheless awesome, constructing pyramids, aqueducts, cathedrals, telescopes, ships that circunnavigated the world, etc.

Now, assuming there is a worldwide disaster that wipes out most of humanity and assuming (a highly unlikely assumption, but anyway…) we have no fossil fuels, then:

(1) At least we can achieve what those ancient people did (and no, we aren’t even near to exterminate all forests in the world).

(2) Given that we can achieve (1), and given that we will have most of our current knowledge (it’s impossible that every written record or piece of technology is destroyed, and every engineer and scientist is dead), then it’s clear that we can recreate a version of our current civilization, with more or less the same medicine, architecture, etc. but without fossil fuels (and no, I’m not referring to wood but nuclear, solar, hydroelectrical… power).

The point is that they won’t be able to leapfrog to hi-tech because they need that base of lo-tech to do it. No fossil fuels, transport is limited. Energy is limited. Can the civilization even mine, extract and purify fissile material or refine silicon for electronic and solar power? Unlikely. We have trouble transitioning and we have the tools and energy. Suppose it took just a millennium to bounce back to the technology of the industrial revolution. Would the records, know-how, and accessible energy be available to extend technology further? Maybe not. Records would be fragmentary. Electronic records are long gone and probably unreadable anyway. Even we cannot make aircraft fly well without fossil fuels, using the most advanced electric storage means we have today. If that bounce-back took 10 millennia, or even longer, then it is effectively starting from scratch. So I don’t buy the argument that you have have a high energy post-industrial world without requiring available fossil fuels to get you there.

I agree to some points you made, I do not think it’s inevitable for life of any kind to go to space. Many might go to technological singularity scenario, end up as lotus eaters in virtual reality worlds. Or their research take them in a direction we have not even thought of yet. (Wild speculation: Lets say they find something odd in the realm smaller than Planck level and start to manipulate the possible against the impossible and get too busy with that to ever go to the stars and disappear on a path at straight angle to reality -haha!)
Others might be happy to create a world large enough to grow in, using up resources of their stellar system to make a Dyson sphere or ring. And then stay put.

My calculation did indeed use the ‘red empire’ I am more pessimistic, to much reading of Lem Stanislaw and Strugatskij in my youth perhaps, but if one hypothesis is correct, then most galaxies are surrounded by a huge halo of red dwarf stars, it is a bit uncertain if that is true, some have looked for infrared excess but see none, then there is infrared from further out – so perhaps it cannot be told apart.
But if there is stars out there also my pessimistic use of Drake Equation will be off by a magnitude.

The entire fossil-fuel-limit argument holds much worse if viewed from a different perspective. Could we reach high-tech if we had no oil at all?

We still couls burn wood. We would reach information age later, but by then, we would already have learn the energy-saving paradigm the hard way. (a grin to all the miners) Solar-generated electricity is a Victorian-age tech if solar-heated steam engines with non-aqueous working fluids are used. Yes, now it seems bulky and not that efficient, but where it would go if it was the only choice?

Those solar steam engines require clear, bright days as they require direct sunlight. That means any facilities would be restricted to those areas where the climate is suitable. However, that would allow synthetic fuels to be manufactured for other purposes, like heating and transport. Solar technology would look like the examples in books from the 1970s when there was a fuel crisis and silicon cell technology was out of reach except for satellites.

I disagree, one asteroid or meteorite and even micro-meteorites would destroy anything in space over a long period of time. On earth an atmosphere protect us from any such problem except in the most violent planet destroying case. Many land areas on earth are billions of years old, even since the 1960’s we have had enough knowledge of plate tectonics to know what locations on the continent’s would be safe for billions of years.

Knowing exactly what molecules were present could help establish the initial conditions that led to the formation of amino acids and related compounds that came together to form peptides—small protein-like molecules that may have kicked off life on Earth.

“We can look to the asteroids to help us understand what chemistry is possible in the universe,” Hud said in a statement. “It’s important for us to study materials from asteroids and meteorites, the smaller versions of asteroids that fall to Earth, to test the validity of our models for how molecules in them could have helped give rise to life.

“We also need to catalog the molecules from asteroids and meteorites because there might be compounds there that we had not even considered important for starting life,” he added.

Coal is does not have an unlimited supply. It is made from dead animals, microorganism and plants which died in the carboniferous period which includes pennsylvanian and mississippian periods. It’s also accounts for most of the man made greenhouse gases in our atmosphere after 1850. It’s literally a technology that is over 150 years old no matter how much you modernize the plants, they still burn coal which is non renewable energy. It also causes air pollution and toxic wastewater heavy metal pollution which is harmful to humans and aquatic life.

“I get asked if the aliens are evil and want to destroy us. Maybe, but I think in the main they will be peaceful because they have had thousands of years to resolve sectarian, fundamentalist, nationalist questions. However, they still might be dangerous if they simply don’t care about us and we get in the way. In War of the Worlds, the aliens did not hate us. We were simply in the way. In the same way that a developer is a threat to forest animals because he can pave the first, the danger there is from someone who sees that we are just in the way. But for the most part, I think they will be peaceful, but view us like we view forest animals.”

Hmmm, that sounds more like indifference rather than consciously peaceful.

Bob should have *at least* included F-stars as potentially “good stars” as well (if not also mid- to late- A-stars)..

On a more serious note, towards the end, the article focuses on the variability of L (and maybe V) being the dominant factor that can give different answers depending on the assumptions used. However, there are other factors that are just or even more important. For instance, fg, which is the fraction of planets with active biospheres was set to 0.1 without explanation. However, we have no idea what is the fraction of potentially habitable planets (e.g. those located within the habitable zone) on which could life actually arise. We only know that it managed to get started here. Such an estimate for fg is also almost certainly much too high…

The article states:
“and it’s a fair guess that 10 percent of these planetary systems feature a world with an active biosphere”

This is almost certainly not correct. Kepler finds that ~10% (if not slightly less) of relevant stars (F – M) host at least one planet within its star’s HZ. So, this is obviously an upper bound. If 10% of planetary systems host worlds with active biospheres that would imply that ~100% of planets located within the HZ are inhabited. However, we know that isn’t true because Mars in our solar system is in the HZ and it is not habitable. Plus, he assumes that half of these life-bearing planets would be able to evolve complex life, including plants and animals (fm = 0.5). This is also a very optimistic assumption.

At the very least, his value of fg should likely be much, much lower than 0.1.

There is also no discussion in this article about the likelihood that life could arise on a terrestrial planet that is located in the HZ. It does necessarily follow that just because a terrestrial planet is “potentially habitable” that life is likely to arise on it. It could be that life is so hard to get started, even on an HZ planet, that fg is very low, say 0.00001. If that is the case, that would imply a relatively small number of civilizations in the galaxy, possibly explaining the Fermi paradox.

Ultimately, such calculations are pure guesses as we don’t know what the values are for many of these parameters. Kepler was a nice surprise in at least giving us some idea of what fraction of stars host at least one terrestrial planet within their habitable zones, but this information is not nearly enough. We need much more. This particular formulation also introduces even more unknowns than the original Drake Equation, which doesn’t help us constrain the problem any further.

“Any way you slice it, one thing seems rather certain: There’s plenty of them out there.”

Nope. We do not know that either. We can only hope that there are plenty of them out there (so that we can find them with present day and next-generation technologies) and that such civilizations are friendly…

For me, the Drake Equation is misrepresented when referred to as ‘a guide’ to get a feel for how many civs could be out there and is useless when brought into play to answer the Fermi-Question.

It does however have two enormous benefits (aside from being a bit of fun)…
[1] it is incredibly valuable as a way to get people engaged in thinking about the question of ETI and our place in the cosmos, as evidenced by the sheer number of posts in this thread, but more importantly…
[2] it shows us where our knowledge defecits lay. The many parameters are a guide to show us where we must concentrate our efforts to learn more and spurs us on (thanks to Kepler et al we are making headway there and will soon make more with the likes of JWST. Infact, when I was a boy we couldn’t even provide a value for any parameter).

(The other thing it does is remind us not to risk ourselves becoming our own threat by subtly offering us a warning with that ‘lifetime’ parameter, almost offering up a challenge; our longevity isn’t a given so beware).

I personally hope life is very common out there, including elsewhere in the solar-system, and I hope that intelligence that leads to technology isn’t a rarity but we will have to see.

The Drake Equation has endured for so long because it is relatively simple (a linear equation) and focuses on the bedrock of what we consider to be the ingredients for creating and supporting intelligent life. The fact that some of the latter components of the equation have yet to be answered in any concrete way only add to its current existence, somewhat paradoxically. This along with our still rather preliminary and sporadic efforts to actually search for alien life, intelligent and otherwise.

Certainly not, at least to my knowledge. Perhaps if they had and the ‘assumptions’ were more confined it may yield an answer-range that was narrower than “1 to millions”. I agree with what you wrote as that hilights my 2 points above and I wouldn’t dismiss the Drake Equation for precisely those points. Using it to explore the Fermi-Question, specifically, gets us nowhere, however, as two different people discussing the F-Q armed with their hugely differring Drake Eq answers (10 civs versus 10,000,000) won’t make any headway, which was what I was trying to convey with my first paragraph above. It still remains a lovely construction though and despite Dr Zubrins attempts in the main post to improve on it it can’t be beaten (as of yet).

The answer to the Fermi-Question will come not from the Drake Equation, which I suspect will still have unknown parameters at the time we have some hard data from other sources (mentioning JWST again for example) telling us we are/aren’t alone… if we are one of many then that’ll come quicker.

When it comes to calculating how many alien intelligences are in the galaxy, we may have to go with a more complicated formula, but then it will lose its resonance.

There are more than a few reasons why E=mc2 is so popular and well-known even to people who know almost nothing about physics. Its simple and straightforward “design” are high on the list as to why.

The one thing the Drake Equation “suffers” from is being designed from a time when most astronomers and scientists went with what they knew – or thought they knew – about alien life and intelligence: That it would develop in a setup most like ours – and that such life would be far more easy to understand and find if it did go that way.

Case in point: The Drake Equation assumes life will develop on an Earthlike planet circling a yellow dwarf type star. Most SETI efforts were aimed at such systems and in the radio realm at that.

Perhaps instead of changing the original Drake Equation, we could instead keep it for calculating Sol-type stars and Earthlike exoplanets. Instead, we create new variations on the DE for other possible scenarios such as Jovian worlds or beings that evolve from and live on stars themselves.

Now you might gripe that those parameters are currently even more unknowable than the original plan, but guess what, most aspects of the original Drake Equation are also a big pile of guesses, yet that has not led to the equation’s failure or its lack of popularity and discussion/debate.

It would do the field of astrobiology, SETI, METI, etc., a lot of good to have Drake Equations designed for various scientifically plausible scenarios.

GAZETTE: In “The Better Angels of Our Nature,” you explored how the trend toward peace has steadily increased. Why expand the premise of that book for “Enlightenment Now”?

pinker: To my pleasant surprise, war is not the only scourge that has declined over the course of history. Extreme poverty has been decimated: It’s gone from 90 percent of the world’s population to 10 percent. Literacy has increased from about 15 percent to more than 85 percent. Prosperity has increased; longevity has increased from about 30 to about 71 years worldwide, and 80 in the developed world.

Human flourishing has been enhanced in measure after measure, and I wanted to tell the broader story of progress, and also to explain the reasons. The answer, I suggest, is an embrace of the ideals of the Enlightenment: that through knowledge, reason, and science we can enhance human flourishing — if we set that as our goal. The goal, too, is a gift of the Enlightenment, namely the moral commitment to humanism, in which the ultimate good is the well-being of people.

I Built a Stable Planetary System with 416 Planets in the Habitable Zone

Posted By Sean Raymond on Feb 27, 2018

When Frank Drake was a boy, growing up in 1930s Chicago, his parents, observant Baptists, enrolled him in Sunday School. By the time he was 8 years old, he suspected his religion, and others around the world, were, to some extent, environmentally determined—local chance events helped shape them. He began to think the same might be true of civilization, for humans and, perhaps, aliens as well—but he thought it better to keep these thoughts to himself.

But not for long: He would go on to found S.E.T.I., the Search for Extraterrestrial Intelligence, and laid out a simple way to estimate the number of civilizations within our galaxy that we could hope to listen-in on. It’s an equation that looks like this:

Since it was just announced, not much. Dennis Kelly, who wrote the UK thriller Utopia, is doing the adaptation and seems committed to Banks’s utopian vision. Kelly has said: “Far from being the dystopian nightmare that we are used to, Banks creates a kind of flawed paradise, a society truly worth fighting for. Rather than a warning from the future, his books are a beckoning.”

On 23 February this year [2018], the American journal Science published an article by an international group of scientists and prehistorians. It presented a series of dates obtained from layers of calcite that had formed on top of drawings in three Ice-Age-decorated caves in Spain: La Pasiega in the north, Maltravieso in the centre, and Ardales in the south.

The results—c. 64-66,000 years ago—are so early that it makes it certain that Neanderthals must have made these markings on cave walls. There is bound to be a great deal of controversy over these results for a number of reasons.

We have only been dating cave art directly since the 1990s, by the radiocarbon method, which can only date organic material (i.e. charcoal drawings). Back then, we were powerless to date drawings in manganese or ochre (inorganic materials) or engravings.

But now, the calcite-dating method allows us to obtain minimum ages for these other drawings if they have some calcite formed on top of them. The dating method has been used for decades by geologists, but it’s only in recent years that archaeologists have tried to obtain dates for cave art from it—with very exciting results.

The most important evidence emerged during the last few years in France, where a cave called La Roche-Cotard has produced a crude sculpture of stone and bone, dating to c. 70,000 years ago, that represents a kind of face. That cave–which was totally sealed at the end of the Neanderthal period— also contains various patterns of finger-marking on its walls (forming geometric shapes), and some spots of red ochre, proving once again that Neanderthals did mark cave walls.

So these new dates from three Spanish caves, together with the Spanish results already published in 2012, add tremendous grist to the mill. Neanderthals were indeed cave artists long before modern humans adopted the practice.

The capacity for symbolic behaviour, or symbolic thinking, is an important and distinctive part of being human and shows that Neanderthals were indeed human beings and very like us in many different ways. One might even dare speculate that modern humans picked up the idea of producing cave art from the Neanderthals when they encountered them in western Europe!

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last eleven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

Centauri Dreams publishes selected comments on the articles under discussion here. The primary criterion is that comments contribute meaningfully to the debate. Among other criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations. A long form of the policy can be viewed on the Administrative page. The short form is this: If your comment is not on topic and respectful to your fellow readers, I'm probably not going to run it.